The present invention relates to the tread of a highway tire having a radial carcass reinforcement and a tread reinforcement which is particularly suited for travel on snow-covered, icy or wet ground.
The tread of the present invention is generally formed of elements in relief, namely ribs or blocks, which are separated from each other in circumferential direction and/or in transverse direction by grooves and are provided with numerous incisions or slits of a width, other than zero, which is far less than the width of the circumferential and transverse grooves which define the elements in relief.
The width of an incision, which is generally variable as a function of the size of the tire in question, is between 0.2 mm and 2 mm.
Based on the principal forces exerted by the ground on the relief elements of the tread, the tread generally comprises blocks provided with incisions, the traces of which on the surface of the tread are parallel to each other and the center axes of which are parallel to the center axis of the transverse grooves defining the block. The incisions are active either upon driving torque imposed upon the wheel or upon braking torque. The tread may also have blocks provided with incisions which are parallel to each other and parallel to the circumferential grooves.
The traces on the surface of the tread may be linear, undulated, or zigzag; the center axes of said traces are straight lines indicating the average direction of said traces.
The term "transverse grooves", as used herein, means grooves or portions of grooves, having on the surface of the tread a center axis which forms an angle of between -20.degree. and +20.degree. with the axial direction or direction parallel to the axis of rotation. The term "circumferential grooves", as used herein, means grooves or portions of grooves which form an angle of between -20.degree. and +20.degree. with the circumferential direction of the tire. In the same way, the traces of the incisions will be parallel to each other if the angle formed by said traces is between -20.degree. and +20.degree..
To improve the overall wear of the tread or the adherence to ground of the tire in question, numerous forms of incisions have been proposed. In order to remedy the substantial defect in adherence which is present as from a rate of wear of the tread of between 30 and 55% while decreasing the speed of wear at the beginning of the life of the tire, U.S. Pat. No. 4,994,126 proposes incisions which, in cross section, form traces in the shape of forks having at least two branches, which branches may have the general shape of either a U or a V.
The compromise between the adherence properties and the wear properties obtained with this type of incision is very good. However, it can be improved, in particular, by a substantial gain in adherence on ground of the tire in question while permitting very acceptable road behavior.